It is already known that motor vehicles equipped with an Otto engine can be operated with a natural gas, known as CNG (Compressed Natural Gas). The CNG mixture is also known as natural gas. With the appropriate equipment, such a CNG motor vehicle can be operated either exclusively with natural gas (monovalent operation) or as a bi-fuel variant with the option of gasoline or natural gas operation. The natural gas is heavily compressed at high pressure of around 200 bar and fed into one or more pressurized fuel tanks in the motor vehicle. The main component of natural gas is methane (CH4) with 85-98%. In addition however natural gas also contains significant proportions of higher hydrocarbons such as ethane (C2H6), propane (C3H8) and butane (C4H10). This is then referred to as a wet natural gas. The reason for this lies in the fact that the ethane, propane and butane components have a relatively low vapor pressure and thus vaporize quickly under pressure. The vapor pressure at 20° C. amounts to around 38 bar for ethane, 8.5 bar for propane and 2.0 bar for butane. Methane, the main component of the natural gas, only has a vapor pressure of 1.47 bar at a temperature of minus 157° C.
If the fuel tank is filled with the CNG mixture at high pressure, it is essentially methane that is available in a gaseous form, with the ethane, propane and butane components mostly being present in a liquid phase. These liquid components collect on the floor of the fuel tank and are not used while the gas pressure in the fuel tank is greater than the vapor pressure of ethane, propane or butane. If on the other hand the gas pressure in the fuel tank reaches the value of the vapor pressure of ethane, then the liquid ethane proportion evaporates first, for which the vapor pressure at 20° C. lies at around 38 bar. If the gas pressure in the fuel tank falls further, then at 8.5 bar the propane proportion and finally at 2 bar the butane proportion evaporate. The result of this physical behavior is that when the gas mixture is injected into the internal combustion engine in conjunction with the induced air, the chemical composition of the gas mixture changes continuously. With a full fuel tank a pure methane-air mixture is injected or burned, with the gas pressure in the fuel tank (system pressure) falling continuously. When the vapor pressure of ethane is reached at around 38 bar, this begins to evaporate and a mixture of methane and ethane is produced. The system pressure remains constant until such time as the ethane proportion in the fuel tank is evaporated. Subsequently the system pressure falls further. When the system pressure reaches the vaporization threshold of propane at around 8.5 bar. the liquid propane proportion evaporates. As from this point a fuel mixture of methane, ethane and propane is burned.
As the tank is emptied further, the butane proportion also evaporates at appr. 2 bar. In practice the latter situation will hardly ever occur since as a rule the injection pressure in the cylinder of the internal combustion engine is driven to over 2 bar and thus the butane proportion remains liquid and in such cases accumulates continuously in the fuel tank.
Since the different components of the CNG have different energy contents, significant effects emerge for the operating behavior of the internal combustion engine in relation to the cylinder filling, the mixture formation, the duration of the fuel injection and the combustion. The exhaust gas emissions can also be especially influenced by this.